In a CDMA (Code Division Multiple Access) system, signals of multiple users are overlapped in time domain and frequency domain. Since signals of different users are correlated, signals of different users at the receiver will create some interference and this kind of interference is called MAI (multiple access interference). Although the MAI of one user is small, with an increase in user number or signal power, the MAI will become one of the most important interferences for the CDMA system.
The TD-SCDMA system may eliminate the above multiple access interference by the multi-user joint-detection technology to increase the anti-interference ability of the system. The multi-user joint-detection refers to that when the receiver separates the received signals, it doesn't regard the MAI as the interference signal but fully makes use of the prior information included in the MAI such as the channel impulse response of each user, and separates signals of all the users from the received signals. Through the above multi-user joint-detection, the anti-interference ability of the TD-SCDMA system can be greatly improved and the capacity and coverage of the system is improved.
One premise to perform the multi-user joint-detection is to obtain the channel impulse response of each user to be detected. The TD-SCDMA system performs channel estimation according to a midamble transmitted in a normal slot by the user. FIG. 1 shows the sub-frame architecture of the TD-SCDMA system. As shown in FIG. 1, each sub-frame of the TD-SCDMA system includes 7 normal slots (TS0, TS1, . . . , TS6) and 3 special slots (Downlink Pilot Time Slot DwPTS, main guard period for TDD operation GP and uplink Pilot Time Slot DwPTS). Each normal slot includes two data domains and one midamble domain, wherein the user transmits service data in the data domain and transmits midamble allocated by the system in the midamble domain of the uplink slot so that the base station may perform the channel estimation and synchronous control. The TD-SCDMA system allocates one basic midamble for each cell and different cells use different basic midambles, which are orthogonal to some extent. Midambles used by different users in the same cell are obtained by cyclically shifting the basic midamble for the cell and cyclical shifts for different users are different. Therefore, the base station may estimate the channel impulse responses of all the users in the cell once according to the basic midamble for the cell and by distinguishing midambles transmitted by users of the cell and the neighboring cells with correlation computations. Since midambles used by different users in the same cell have different cyclical shifts, the channel impulse responses of different users in the same cell may have different delays, i.e. channel impulse responses of different users in the same cell will be in different channel estimation windows. The position of the channel estimation window of each user may be determined by the cyclical shift of the midamble used by the user.
After the position of the channel estimation window of each user is determined, the base station may intercept the channel impulse response of each user from those of all the users in the cell and perform the multi-user joint-detection according to the channel impulse response of each user.
The above multi-user joint-detection process shows currently the base station can only perform the multi-user joint-detection on uplink signals of all the users in the cell and regard uplink signals of users in neighboring cells as unknown multiple access interference. Therefore, the above multi-user joint-detection method can only suppress the multiple access interference between users in the cell and can cannot eliminate the multiple access interference from neighboring cells.
The influence of the multiple access interference from neighboring cell users on the system performance in the case of co-frequency networking will be illustrated with reference to FIG. 2. In the three cells shown in FIG. 2, all the users use the same frequency and slot resources. As shown in FIG. 2, when a user equipment (UE) 21 of cell 2 moves from cell 2 to cell 1, in order to guarantee the communication quality, UE21 will gradually increase its transmitting power under the uplink power control so that Node B1 of cell 1 receives stronger and stronger uplink signal power of UE21. For all the users in cell 1, uplink signals transmitted by UE21 will create stronger and stronger multiple access interference. In order to suppress the interference from the uplink signals of UE21, users in cell 1 will increase their transmitting power under the uplink power control so as to guarantee the communication quality. Thus, uplink signals of users in cell 1 will create greater multiple access interference for neighboring cells such as cell 2 and cell 3. Therefore, users in cell 2 and cell 3 such as UE32 will further increase its transmitting power. In this way, a positive feedback is formed and the average interference level of the whole network continues to increase. Since the transmitting power of a user is limited, the system capacity and coverage will be reduced finally.
According to the principle of the multi-user joint-detection technology, if Node B can obtain resources such as scrambling code, code channel, basic midamble and channel impulse response used by users of co-frequency neighboring cells which create the multiple access interference for the present cell, Node B may perform a unified multi-user joint-detection on received uplink signals of neighboring cells and of the present cell so as to effectively reduce the multiple access interference on users of the present cell from neighboring cells, further increase the uplink anti-interference ability of users of the present cell and increase the system capacity and coverage.
Node B may obtain information such as scrambling code, code channel and basic midamble used by neighboring cells from configuration information of the system. Therefore, one of the key problems is to obtain accurate channel impulse responses of users of neighboring cells so that uplink users of the neighboring cells may participate in the multi-user joint-detection. However, in the TD-SCDMA system, each user keeps synchronous with its cell, not necessarily with neighboring cells so Node B may easily locate positions of channel estimation windows of all the users in the cell and further obtain channel impulse responses of all the users in the cell but it cannot accurately locate positions of channel estimation windows of users in neighboring cells and therefore cannot obtain reliable uplink channel impulse responses of users in neighboring cells. If wrong channel impulse responses are used in the multi-user joint-detection process, greater interference will be created for users in the cell.